Neutral chiral cyclopentadienyl Ru(ii)Cl catalysts enable enantioselective [2+2]-cycloadditions

Cyclopentadienyl ruthenium(ii) complexes with a large number of available coordination sites are frequently used catalysts for a broad range of transformations. To be able to render these transformations enantioselective, we have designed a chiral neutral Cp^xRu(ii)Cl complex basing on an atropchiral cyclopentadienyl (Cp^x) ligand which is accessed in a streamlined C-H functionalisation approach. The catalyst displays excellent levels of reactivity and enantioselectivity for enantioselective [2+2]-cycloadditions leading to strained chiral cyclobutenes, allowing for catalyst loadings as low as 1 mol%. A very strong counterion effect of a bound chloride anion transforms the corresponding unselective cationic complex into a highly enantioselective neutral version. Moreover, by adding norbornadiene at the end of the reaction the catalyst can be recovered and subsequently reused.